Rig pose of proxy armature isn't saved
This is due to protected layers on the armature. https://docs.blender.org/manual/en/latest/animation/armatures/properties/skeleton.html#protected-layers
Also, keyframing your poses is always a good idea, and quite the opposite of hacky. If you aren't keyframing your poses you're hanging by a thread, and opening yourself up to various ways of losing said posing, this being only one of them!